This invention relates to gas analyzers. In particular, the invention concerns non-dispersive infrared gas analyzers. The invention is especially adapted for measuring the concentration of gas components in vehicle exhaust gas.
Non-dispersive Infrared (IR) gas analyzers utilize an IR source to direct IR radiation through a mixture of gases contained in a sample chamber. The IR energy is passed through the mixture in the sample chamber at absorption frequencies for gases whose concentration is to be determined. The detected absorption at each frequency is indicative of the concentration of the component gas having the particular absorption band. In the particular application to automotive gas analyzers, the gases whose concentrations are of interest include HC (hydrocarbons), CO and CO.sub.2. In order to measure the concentration of these gases, multiple light filters, having transmission bands at an absorption band for each component gas, are alternatingly placed between the source and detector to provide an indication for each gas. The detector output is a single, time-multiplexed signal which contains information for all component gas concentrations. This signal is conventionally demultiplexed into individual signals and applied to separate amplifier channels for each gas component.
To produce accurate readings of automotive exhaust gases, the system must be calibrated frequently in order to make adjustments for drift in the components as well as for the buildup of exhaust particles on surfaces of optical components. Because of the significant decreases in allowed vehicle emission of pollutants, more sensitive and accurate measurements are required. At least one state now requires that the gain of each gas-measurement channel be calibrated at a minimum of two points with respect to full scale, or 100% span. For example, the two calibration points may be at 20% and 60% of full scale. Overall performance of the gas analyzer must be superior to meet these requirements, as well as those that are likely to be imposed in the future.
The output of each gas channel additionally has a tendency to drift due to temperature variations, component aging and other factors. This drift, or offset, must be cancelled, or zeroed, in order to avoid false readings. Because each gas measuring channel must be individually zeroed, the required effort, and chances for error, are multiplied. While automated, self-zeroing schemes have been proposed, such schemes typically merely automate functions that were previously manually performed. The result is yet a further complication of the hardware and increased chances for error.